U.S. patent number 6,198,928 [Application Number 09/043,012] was granted by the patent office on 2001-03-06 for handover method, and a cellular radio system.
This patent grant is currently assigned to Nokia Telecommunications Oy. Invention is credited to Hannu Hakkinen, Seppo Hamalainen, Jere Keurulainen.
United States Patent |
6,198,928 |
Keurulainen , et
al. |
March 6, 2001 |
Handover method, and a cellular radio system
Abstract
A cellular radio system and a handover method in a cellular
radio system is provided. The cellular radio system includes a
subscriber terminal and a base station. In the system, the
subscriber terminal maintains and continuously updates an active
set of base stations. One or several base stations belonging to the
active set has a connection or connection set-up facilities to the
subscriber terminal. The subscriber terminal controls the downlink
transmission of one or several base stations of the active set by
switching the transmission of the base station on or off. This
method provides a fast soft handover similar to a hard
handover.
Inventors: |
Keurulainen; Jere (Helsinki,
FI), Hakkinen; Hannu (Espoo, FI),
Hamalainen; Seppo (Espoo, FI) |
Assignee: |
Nokia Telecommunications Oy
(Espoo, FI)
|
Family
ID: |
8556624 |
Appl.
No.: |
09/043,012 |
Filed: |
March 2, 1998 |
PCT
Filed: |
August 31, 1995 |
PCT No.: |
PCT/FI95/00467 |
371
Date: |
March 02, 1998 |
102(e)
Date: |
March 02, 1998 |
PCT
Pub. No.: |
WO97/08911 |
PCT
Pub. Date: |
March 06, 1997 |
Current U.S.
Class: |
455/436; 455/437;
455/442 |
Current CPC
Class: |
H04W
36/36 (20130101) |
Current International
Class: |
H04Q
7/38 (20060101); H04Q 007/38 () |
Field of
Search: |
;455/436,437,442,59,86,315,522 ;370/331 ;375/200 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
952396 |
|
Nov 1996 |
|
FI |
|
95/12297 |
|
May 1995 |
|
WO |
|
Primary Examiner: Bost; Dwayne
Assistant Examiner: Trinh; Sonny
Attorney, Agent or Firm: Pillsbury Winthrop LLP Intellectual
Property
Parent Case Text
This application is the national phase of international application
PCT/F195/00467 filed Aug. 31, 1995 which designated the U.S.
Claims
What is claimed is:
1. A handover method in a cellular radio system including a
subscriber terminal and a plurality of base stations, the method
comprising:
maintaining and continuously updating, by the subscriber terminal,
an active set of the base stations, at least one of the base
stations belonging to the active set having a respective connection
to the subscriber terminal; and
deciding, by the subscriber terminal, to switch on or off a
respective downlink transmission of the at least one base station
of the active set.
2. A method according to claim 1, wherein the subscriber terminal
transmits a command signal ordering the at least one base station
of the active set to switch on the respective downlink
transmission.
3. A method according to claim 1, wherein the subscriber terminal
transmits a command signal ordering the at least one base station
of the active set to switch off the respective downlink
transmission.
4. A method according to claim 1, wherein the subscriber terminal
decides to switch on or off the respective downlink transmission of
the at least one base station of the active set by controlling
signaling and a traffic channel in such a way that the at least one
base station of the active set loses the respective connection to
the subscriber terminal, whereupon the at least one base station of
the active set switches off the respective downlink
transmission.
5. A method according to claim 1, wherein the at least one base
station of the active set switches off the respective connection
with the subscriber terminal after a predetermined delay.
6. A method according to claim 1, wherein the at least one base
station of the active set switches off the respective connection
with the subscriber terminal by reducing transmit power within a
predetermined period of time.
7. A method according to claim 2, wherein the subscriber terminal
transmits a command signal separately for each base station.
8. A method according to claim 2, wherein the command signal is
common to all base stations of the active set.
9. A method according to claim 2, wherein the subscriber terminal
transmits a command signal at regular intervals.
10. A method according to claim 9, wherein a transmission of
traffic channels takes place at regular intervals in a form of
frames and the subscriber terminal transmits the command signal at
intervals of a frame.
11. A method according to claim 2, wherein when the command signal
intended for the at least one base station of the active set is
divided in fragments received by different base stations, the
command signal intended for the at least one base station is
assembled in a base station controller or in some part of the
network from said fragments, and the command signal formed in this
manner is signaled back to the at least one base station.
12. A cellular radio system comprising:
a subscriber terminal; and
a plurality of base stations, the subscriber terminal maintaining
and continuously updating an active set of the base stations, at
least one of the base stations belonging to the active set and
having a connection to the subscriber terminal, wherein
the subscriber terminal comprises:
means for deciding to switch on or off a downlink transmission of
at least one base station of the active set.
13. A cellular radio system according to claim 12, wherein the
subscriber terminal comprises means for forming and transmitting a
command signal ordering the at least one base station of the active
set to switch on a transmission.
14. A cellular radio system according to claim 12, wherein the
subscriber terminal comprises means for forming and transmitting a
command signal ordering the at least one base station of the active
set to switch off a transmission.
15. A cellular radio system according to claim 12, wherein the
means for deciding to switch on or off downlink transmission of at
least one base station of the active set is arranged to control
signaling and traffic channels of the subscriber terminal in such a
way that the at least one base station of the active set loses the
connection to the subscriber terminal, and at least one of the base
stations comprises means for switching off a respective downlink
transmission when the at least one of the base station loses a
respective connection to the subscriber terminal.
16. A cellular radio system according to claim 12, wherein the base
stations, each comprise means for switching off a respective
transmission after a predetermined delay.
17. A cellular radio system according to claim 12, wherein the base
stations, each comprise means for switching off a respective
transmission by reducing transmit power within a predetermined
period of time.
18. A cellular radio system according to claim 13, wherein the
subscriber terminal comprises means for transmitting a command
signal separately for each of the base stations.
19. A cellular radio system according to claim 13, wherein the
subscriber terminal comprises means for transmitting a command
signal that is common to all of the base stations of the active
set.
20. A cellular radio system according to claim 12, wherein the
subscriber terminal comprises means for transmitting a command
signal at regular intervals.
21. A cellular radio system according to claim 20, wherein
transmission of traffic channels takes place at regular intervals
in a form of frames and the subscriber terminal comprises means for
transmitting a command signal at intervals of a frame.
22. A subscriber terminal for a cellular radio system, wherein the
subscriber terminal comprises:
means for maintaining and continually updating an active set of
base stations; and
means for deciding to switch on or off a respective downlink of at
least one of the base stations of the active set.
23. The subscriber terminal according to claim 22, wherein said
means for deciding to switch on or off a a respective downlink of
at least one of the base stations of the active set comprises means
for transmitting a command signal to order the at least one base
station of the active set to switch on the respective downlink
transmission.
24. The subscriber terminal according to claim 22, wherein said
means for deciding to switch on or off a a respective downlink of
at least one of the base stations of the active set comprises means
for transmitting a command signal to order the at least one base
station of the active set to switch off the respective downlink
transmission.
25. The subscriber terminal according to claim 22, wherein said
means for deciding to switch on or off a a respective downlink of
at least one of the base stations of the active set comprises means
for controlling signaling and a traffic channel such that the at
least one base station of the active set loses a respective
connection to the subscriber terminal and the at least one base
station of the active set switches off the respective downlink
transmission.
26. A method according to claim 3, wherein the at least one base
station of the active set switches off the respective connection
with the subscriber terminal after a predetermined delay.
27. A method according to claim 4, wherein the at least one base
station of the active set switches off the respective connection
with the subscriber terminal after a predetermined delay.
28. A method according to claim 3 wherein the at least one base
station of the active set switches off the respective connection
with the subscriber terminal by reducing transmit power within a
predetermined period of time.
29. A method according to claim 4 wherein the at least one base
station of the active set switches off the respective connection
with the subscriber terminal by reducing transmit power within a
predetermined period of time.
30. A method according to claim 3, wherein the subscriber terminal
transmits a command signal separately for each base station.
31. A method according to claim 3, wherein when the command signal
intended for the at least one base station of the active set is
divided in fragments received by different base stations, the
command signal intended for the at least one base station is
assembled in a base station controller or in some part of the
network from said fragments, and the command signal formed in this
manner is signaled back to the at least one base station.
32. A cellular radio system according to claim 14, wherein the base
stations, each comprise means for switching off a respective
transmission after a predetermined delay.
33. A cellular radio system according to claim 15, wherein the base
stations, each comprise means for switching off a respective
transmission after a predetermined delay.
34. A cellular radio system according to claim 14, wherein the base
stations, each comprise means for switching off a respective
transmission by reducing transmit power within a predetermined
period of time.
35. A cellular radio system according to claim 15, wherein the base
stations, each comprise means for switching off a respective
transmission by reducing transmit power within a predetermined
period of time.
36. A cellular radio system according to claim 14, wherein the
subscriber terminal comprises means for transmitting a command
signal separately for each of the base stations.
37. A cellular radio system according to claim 14, wherein the
subscriber terminal comprises means for transmitting a command
signal that is common to all of the base stations of the active
set.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a handover method in a cellular radio
system comprising at least one subscriber terminal and a base
station, and in which system the subscriber terminal maintains and
continuously updates an active set of base stations, one or several
base stations belonging to the active set having a connection or
connection set-up facilities to the subscriber terminal.
The invention also relates to a cellular radio system comprising at
least one subscriber terminal and a base station, in which system
the subscriber terminal maintains and continuously updates an
active set of base stations, one or several base stations belonging
to the active set having a connection or connection set-up
facilities to the subscriber terminal.
2. Description of Related Art
When a subscriber terminal moves from one cell to another in a
cellular radio system, a handover is performed, and this handover
is either, in a simple case, a hard handover or a more flexible
soft handover. The drawback of the hard handover is that the old
connection is broken before a new one is set up. The problem is
more marked especially in the ping-ponging effect wherein a channel
keeps changing back and forth between different channels. This can
be diminished by using a handover margin. However, the best base
station connection cannot be utilized in such a case due to the
handover margin and the delay of the handover procedure, wherefore
an unnecessarily high transmit power must be used and the
disturbance power of the system thereby increases.
The prior art soft handover also employs updating margins, which
are covered, however, in the reception of the subscriber terminal
by signals from base stations with better audibility. Furthermore,
even these unnecessary signals increase the transmit power of the
base stations and interfere with the other subscriber terminals.
The prior art handover is described for example in Finnish Patent
Application 952,396 (Granlund, Hakkinen, Hamalainen): "Method for
improving the reliability of handover and call establishment, and a
cellular radio system", which is incorporated herein by
reference.
As it is known, a pilot signal is used to identify a base station
and to form an active set in the CDMA system. A pilot signal is a
data-unmodulated spreading-coded signal, which is continuously
transmitted by each base station to its coverage area. A terminal
equipment can identify the base stations on the basis of the pilot
signal, since the spreading codes of the pilot signals differ from
one another.
Subscriber terminals continuously measure pilot signals. In order
to reduce the measurement load of a terminal equipment in prior art
systems, each terminal equipment maintains a measurement list of
the base stations and the corresponding spreading codes of the
pilot signals that are situated near the terminal equipment and
that are possible candidates for handover or connection
establishment. The base stations on the measurement list form a
group of candidates, which may become members of the active set. It
is possible to establish connections rapidly to the active set from
the fixed network. Terminal equipments monitor with the highest
priority the pilot signals of only those base stations that are on
the measurement list.
When a terminal equipment moves, the measurement list must
naturally be updated as the need arises. In the prior art systems
updating is performed according to the measurement performed by the
terminal equipment on the strength of the pilot signal, i.e. if a
pilot transmitted by a base station is received with adequate
strength, it is added to the measurement list.
A rake receiver used in particular in the CDMA system comprises
several branches, each of which may be synchronized with a
different signal component. The receiver can therefore receive
several signals simultaneously. On the basis of the measurements of
the pilot signal, the branches of the rake receiver are also caused
to receive signals that arrive along different propagation paths.
The rake receiver adapts to the attenuation changes over the
different connections considerably faster than the active set is
updated.
In a CDMA network providing many services there occur, however,
situations wherein the load of the base station in the transmission
direction from the terminal equipment to the base station, i.e. in
the uplink transmission direction, is considerably greater than in
the opposite transmission direction. An example of this is the
unidirectional data transmission from the terminal equipment to the
network. The prior art arrangements for updating the measurement
list do not detect and therefore change the load of the base
station in this transmission direction.
Even though the best signal can be selected and the power of
signals which have propagated along different paths can be compiled
when a rake receiver is used in the subscriber terminal, the number
of the rake branches is limited by the power consumption and the
manufacturing costs, and therefore the rake receiver cannot
utilize, however, more than a few transmissions at a time.
Neither do the known systems provide the possibility of
transferring connections to other base stations in a situation
where an individual base station is overloaded.
SUMMARY OF THE INVENTION
The purpose of the present invention is to avoid the problems of
the known arrangements and to implement a soft handover in the
manner of hard handover with a very small effective handover
margin, and to decrease the disturbance power of the base station
when the subscriber terminal communicates with several base
stations.
This is achieved with the method of the type described in the
preamble, characterized in that the subscriber terminal controls
the downlink transmission of one or several base stations of the
active set in such a way that each base station switches its
transmission on or off.
The cellular radio system according to the invention is
characterized in that the subscriber terminal comprises means for
controlling one or several base stations of the active set in such
a way that each base station switches its downlink transmission on
or off.
The invention provides considerable advantages. The number of
signals transmitted by the base stations can be decreased, and the
interference level caused by the base stations can thereby be
reduced. This in turn increases the capacity of the system and
improves the quality of the connections.
Handover also becomes faster. The updating of the active set is
slow and requires a great deal of signalling. According to the
invention, a base station can be activated by means of the internal
signalling of the radio interface or alternatively by means of
routed signalling. A mobile station has to keep active only 1 to 3
base stations providing the best connection, and to update this
group rapidly by means of the signalling according to the
invention. The result is a process, similar to a hard handover,
from the base station to the terminal equipment with a very small
effective handover margin.
The fixed network may operate in the same manner as in a
conventional macro diversity system, and the arrangements according
to the invention are only directed at the radio interface.
The invention enables the size of the active set and the selection
margin to be increased without increasing the interference. This
facilitates the maintenance of the best connection especially under
difficult circumstances in a microcell environment.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in greater detail below with
reference to the examples according to the accompanying drawings,
in which
FIG. 1 shows a cellular radio system,
FIG. 2 shows the essential parts of a subscriber terminal, and
FIG. 3 shows the essential parts of a base station.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method according to the invention will be described in greater
detail below. In the method according to the invention, the
selection of the macro diversity active set is based on the
comparison of pilot signals, transmitted by the base stations, in
the subscriber terminal. When the power of a pilot signal of a base
station received at a subscriber terminal comes close to the power
of the strongest received pilot signal within a certain margin,
this base station is added to the active set of base stations
maintained by the subscriber terminal. Otherwise the base station
is removed from the active set when its signal deteriorates. The
base stations of the active set are synchronized and receive the
signal of the subscriber terminal if the signal-to-interference
ratio enables it. Connections are established from the base
stations through the fixed network, and through these connections
the signals received by the base stations can be combined. The
signal of the base stations of the active set to be transmitted to
the subscriber terminal is transferred in a similar manner. The
base stations of the active set have connections or at least
connection set-up facilities to the subscriber terminals.
By means of measurements of signal quality, a subscriber terminal
can change the transmission of the base stations. The subscriber
terminal controls the downlink transmission of the base stations by
switching the transmission on or off. For example, the subscriber
terminal can decide to switch on or off the transmission of a base
station actively and autonomously. The subscriber terminal
transmits a command signal that orders at least one base station of
the active set to switch its downlink transmission on or off. When
the base station transmission is switched off in the arrangement
according to the invention, the base station transmission is
completely terminated or the transmit power is decreased by a
desired degree, so that the interference caused by the base station
disappears. The decrease in the transmit power can be for example
between 40 dB and 3 dB. Switching a base station transmission on
and off is preferably performed by means of a command signal, which
can be received by the base station directly from the transmission
of the subscriber terminal, or the command signal can be forwarded
by the fixed network. Especially the command signal that switches
on the base station transmission is either common to all the base
stations of the active set, or it is separate for each base
station, and this command signal is transmitted regularly,
preferably at intervals of a frame. Transmitting the command signal
regularly provides the advantage that the transmit power of the
base stations can be adjusted at precise intervals, and when the
interval between the adjustments is short, for example one frame,
the adjustment is performed rapidly enough to observe even fast
changes in the attenuation over the connection. All base stations
receiving the command signal perform the power adjustment. The base
stations which cannot receive the signal of the subscriber terminal
observe the transmission error and terminate the transmission to
this subscriber terminal. The transmission is terminated most
preferably by decreasing the transmit power slowly within a
predetermined period of time or by disconnecting the transmission
after a predetermined delay. When the base station terminates the
transmission after it has lost the connection, i.e. after the
connection has deteriorated below a predetermined level, the
advantage is that the number of the base stations communicating
with the subscriber terminal can be restricted to the smallest
possible. The base station starts retransmitting when it can
receive the command signal of the subscriber terminal switching on
the base station.
In another preferred embodiment of the invention, the subscriber
terminal transmits a command signal by means of which the
transmission of each base station of the active set is separately
adjusted. Each of the base stations can separately receive the
command signal, so that each base station adjusts its own transmit
power according to what the command signal determines. The base
stations that do not receive the control signal terminate the
transmission to the subscriber terminal in the same manner as in
the first preferred embodiment. The advantage of transmitting the
command signal separately for each base station is that the method
is then more controlled and the power level of each base station
can be separately adjusted to the desired value. The command signal
can also be assembled of signals received by several base stations.
The control signal is then assembled in some part of the network,
for example in the base station controller, and the control signals
are signalled separately to each base station.
FIG. 1 is a diagram of a cellular radio system. The arrangement
according to the invention is applicable in all
interference-limited cellular radio systems, which include for
example different spread spectrum systems, OFDMA systems, and in
the preferred embodiment of the invention, the CDMA system. The
cellular radio system comprises a number of subscriber terminals
101 and 102, and base stations 103 to 107. The subscriber terminals
101 and 102 communicate with the base stations 103 to 107 on
certain traffic channels when the subscriber terminals are situated
within the coverage area of the base stations. When the CDMA system
is used, a traffic channel consists of a wide frequency band used
by all terminal equipments 101 and 102 as they transmit to the base
station 103 to 107 and also used by the base stations 103 to 107 in
the downlink transmission direction. It is assumed in FIG. 1 that
the subscriber terminal 101 communicates with the base stations 103
and 104, but that it is in the process of shifting the connection
from the base station 104 to the base station 105, since the
connection with the base station 104 deteriorates and the
connection with the base station 105 is improved. The terminal
equipment 101 transmits a command signal 110, which connects the
base stations to transmit in the downlink transmission direction.
In this example, the base stations 103, 104 and 105 form the active
set of the subscriber terminal 101, since the quality of the
connection to these base stations is better than the required
signal noise level. Since the connection to the base station 105 is
improving, which can be seen on the basis of the measurement on the
pilot signal, the base station 105 then receives the control and
continues the connection with the terminal equipment 101. The
connection to the base station 104 deteriorates further and the
base station finally loses the command signal 110. According to the
arrangement of the invention, the base station then terminates the
connection to the terminal equipment 101. The subscriber terminal
may also transmit a command signal by means of which the downlink
transmission of the base station is switched off. This command
signal is preferably transmitted separately for each base station,
so that not all the base stations of the active set terminate their
transmission.
A base station controller 108 manages the control of the base
stations 103 to 106 and assembles the signals arriving at the base
stations 103 to 106 when the different parts of the signal have
been received at different base stations. The base station
controller 108 operates in this manner especially when, in an
arrangement of the invention, the subscriber terminal 101 transmits
the command signal 110 regulating the transmit power of the base
stations in such a way that the different parts of the signal are
received at different base stations. The base station controller
108 forwards the message of the command signal it has assembled via
the fixed network 109 to the base stations 103 to 106, which
operate according to the command of the control signal 110.
FIG. 2 illustrates a transceiver of a subscriber terminal in the
cellular radio system, the method according to the invention being
applicable in such a transceiver. The receiving part of the
subscriber terminal comprises a receiver unit 216 receiving a
signal that is converted to an intermediate frequency, and
converter means 218 in which the signal is converted to a digital
form. The converted signal is supplied to detector means 220 from
where the detected signal is supplied further to de-interleaving
means 222 and to means 224 wherein the received signal is decoded,
i.e. subjected to both channel and speech decoding.
The transmitting part of the subscriber terminal further comprises
means 200 for encoding the signal to be transmitted. The encoded
signal is supplied to means 202 for interleaving the encoded
signal. The output signal of the interleaving means is connected to
the input of the means 204, where the burst to be transmitted is
formed. The resulting signal is supplied to modulation means 208,
the output signal of which is supplied via a transmitter unit 210
and a duplex filter 212 to an antenna 214. The aforementioned
blocks can be implemented in known manners.
The apparatus further comprises control and counting means 226,
which control the operation of the other aforementioned blocks. The
control means 226 may be used to adjust the transmit power of the
subscriber terminal itself, i.e. the signalling and traffic
channels of the subscriber terminal can be adjusted. In the
arrangement according to the invention this concerns especially the
command signal adjusting the transmit power of the base stations.
When the power of the command signal is regulated, the number of
the base stations situated within the coverage area of the
subscriber terminal can be restricted. This provides the advantage
that the subscriber terminal can keep active only 1 to 3 base
stations providing the best connection, and update this group
rapidly by means of signalling according to the invention. This
results advantageously in operation similar to a hard handover in
the downlink transmission direction with a very small effective
handover margin. The control means 226 receive and compares signals
from different base stations. On the basis of the comparison, means
226 decides whether to switch on or off a downlink transmission of
at least one base station of the active set. Means 226a comprised
by the command unit forms a command signal or control signals
adjusting the transmit power of the base stations, the signal(s)
being transmitted normally via the transmitting part. Adjusting the
transmission of the base stations provides the advantage that the
interference level caused by the base stations can be reduced,
since the total transmit power of several base stations can be
optimized to suit each situation. In the arrangement according to
the invention, the transmission of the base stations is adjusted in
such a way that the control signal switches the base station
transmission on or off. When the base station transmission is
disconnected, its transmit power is entirely switched off or the
transmit power has been reduced by a certain amount, for example 20
dB. The advantage provided by this procedure is a fast handover.
The control means 226 can also be used to adjust the transmit power
of the terminal equipment itself, the advantage being that the
number of the base stations with which the terminal equipment
communicates can be restricted and the number of the interference
signals can be simultaneously reduced. The control means 226 and
226a of the subscriber terminal are typically realized by means of
a processor, but they may also be realized with several other kinds
of electronic connections, which may perform similar functions as a
processor. The subscriber terminal also comprises means 220, 226
for measuring the strength of a pilot signal it has received from a
base station. If the system is a CDMA system, the detector block in
the receivers, usually realized according to the rake principle,
typically comprises several receiver branches at least one of which
is a so-called searcher branch, which measures the strengths of the
pilot signals.
FIG. 3 is a diagram illustrating a transceiver of a base station.
The transceiver of the base station is almost identical to the
transceiver of the subscriber terminal. The transceiver of the
cellular radio system according to the invention comprises means
220, 226 for estimating the attenuation over the connection between
the terminal equipment and the base station by means of the
received power of the pilot signal and the power used in the
transmission. The transceiver of the base station further comprises
means 220, 226 for estimating the signal-to-noise ratio of the
uplink transmission direction on the basis of the total
interference, the attenuation over the connection, and the transmit
power of the terminal equipment. The base station equipment also
comprises means 220, 226 for measuring the total interference of
the signal it has received from the terminal equipments, and means
220 to 214, 226 for transmitting a pilot signal with known transmit
power.
The greatest difference between the arrangement according to the
invention and the prior art is in the control means 226. The
control means 226 of the base station transceiver in the
arrangement of the invention comprises means 226b for initiating a
transmission to the subscriber terminal when the base station
identifies the command signal of the subscriber terminal, and with
the same means 226b the base station terminates the transmission to
the subscriber terminal when the base station no longer identifies
the command signal transmitted by the subscriber terminal to switch
on the transmission, or when the base station receives a command
signal by means of which the transmission is switched off. It is
possible to terminate the transmission slowly with the means 226b
and to avoid an immediate disconnection by reducing the transmit
power of the base station within a predetermined delay. Terminating
the transmission when the command signal is no longer clear reduces
the number of the base stations communicating with a particular
subscriber terminal, thus decreasing interference. Avoiding an
immediate connection breakdown provides the advantage that the
connection between the base station and the subscriber terminal is
not lost due to a temporary deterioration in the
signal-to-interference ratio. The control means 226 and 226b of the
base station are typically realized by means of a processor, but
they may also be realized with several other kinds of electronic
connections, which may perform similar functions as a
processor.
Even though the invention is described above with reference to the
example according to the accompanying drawings, it is clear that
the invention is not restricted thereto, but it can be modified in
many ways according to the inventive idea disclosed in the appended
claims.
* * * * *